Active efforts have been made to develop an engine capable of operating on a lean air-fuel ratio larger than a stoichiometric air-fuel ratio to improve fuel consumption, while also suppressing harmful products included in exhaust gas from the engine. A method of controlling the air-fuel ratio previously proposed in Japanese Application Laid-open No.52825/1978 changes the stoichiometric air-fuel ratio to a lean air-fuel ratio when the operating condition of the engine is within a predetermined range. This method increases the air-fuel ratio gradually after the load on the engine has increased beyond a predetermined value and decreases the air-fuel ratio gradually to an initial air-fuel ratio when the load decreases. Another method previously proposed in Japanese Application Laid-open No.10224/1976 increases the air-fuel ratio when the load on the engine is in a middle-load region, and controls the air-fuel ratio so that a fuel mixture at the stoichiometric air-fuel ratio or a rich air-fuel ratio is supplied to the engine when the load is in a low-load region or a high-load region, attaching importance to the output power of the engine.
Since the fuel amount is reduced below that of the stoichiometric air-fuel ratio during operation at a lean air-fuel ratio, the engine torque decreases when the air-fuel ratio is increased with the opening of the throttle valve which has been kept fixed. Since the catalyst for reducing the nitrogen oxide (NOx) concentration in the exhaust gas does not function effectively in a state where the air-fuel ratio is other than the stoichiometric air-fuel ratio, lean-burn operation is possible only at the air-fuel ratio that makes the NOx concentration of the exhaust gas lower than an allowable limit; that is, the transition of the air-fuel ratio between the stoichiometric air-fuel ratio and the air-fuel ratio for lean-burn operation must be completed as quickly as possible to satisfy the requirements for a clean exhaust gas.
It is desirable to use an automatic transmission in combination with such an engine to secure easy driving. The gear changing operation of an automatic transmission is controlled according to a shift schedule stored in a memory of a controller. The shift schedule is designed on the basis of the opening of the throttle valve, which is an index of the engine torque, and the vehicle speed ("Motor Fan", San-ei Shobo, p. 29, Dec., 1990). The lockup schedule also is designed on the basis of the opening of the throttle valve and the vehicle speed. The line pressure is controlled according to the opening of the throttle valve so as to transmit the output power of the engine efficiently to the drive shaft.
As mentioned above, the transition between the stoichiometric air-fuel ratio and the air-fuel ratio for lean-burn operation must be quickly completed. However, since the difference between the torque produced when the engine is operating at the stoichiometric air-fuel ratio and the torque produced when the engine is operating at a lean air-fuel ratio for lean-burn operation is about 30%, the vehicle may be shocked and the driver feels that the output power of the engine is insufficient during lean-burn operation, if the air-fuel ratio is changed quickly.